European Patent No. EP 0 477 400 describes a fuel injector having a path transformer for a piezoelectric actuator in which the actuator transmits a lifting force to a master piston. The master piston is in force-locking connection to a guide cylinder for a slave piston. The slave piston, the guide cylinder and the master piston sealing the guide cylinder form a hydraulic chamber. A spring, which presses the master piston and the slave piston apart, is situated in the hydraulic chamber. Surrounding an end section of the guide cylinder and the slave piston is a rubber sleeve by which a supply chamber for a viscous hydraulic fluid is sealed from a fuel chamber. The viscosity of the hydraulic fluid is adapted to the ring gap between the slave piston and the guide cylinder.
The slave piston mechanically transmits a lifting movement to a valve needle, for example. When the actuator transmits a lifting movement to the master piston and the guide cylinder, the pressure of the hydraulic fluid in the hydraulic chamber transmits this lifting movement to the slave piston, since the hydraulic fluid in the hydraulic chamber is not compressible and during the short duration of a lift only a very small portion of the hydraulic fluid is able to escape through the ring gap into the supply chamber formed by the rubber sleeve. In the rest phase, when the actuator does not exert any pressure on the master piston, the spring pushes the slave piston out of the guide cylinder and the hydraulic fluid, due to the generated vacuum pressure, enters and refills the hydraulic chamber via the ring gap. In this way, the path transformer automatically adapts to longitudinal expansions and pressure-related expansions of a fuel injector.
The sealing by a rubber sleeve, which is pressed against the end section of the guide cylinder and the slave piston by two clamping rings, is unsatisfactory in the long term. The highly viscous hydraulic fluid and the fuel thus may mix over time, and the coupler break down. When gasoline, as one possible fuel, reaches the interior of the coupler, a loss of function may occur since this fluid, due to the low viscosity of the gasoline, may rapidly flow through the ring gap and no pressure is able to be generated in the pressure chamber during the brief dynamic lift duration.